The correction of chromatic aberration requires a lens formed of a high-dispersion glass together with a lens formed of a low-dispersion glass. When aspherical lenses are employed as such lenses, a higher-performance and compact optical system can be realized.
Mass-producing of such lenses requires a glass having a low glass transition temperature, and a phosphate glass is available as a typical example thereof. In addition, a few silica-containing glasses have been also proposed as disclosed in JP 2004-161598A, Re-published WO2004/110942 and JP 2002-87841A.